The present invention relates to a feed-forward amplifier that is used mainly for high-frequency power amplification in multi-carrier radio communications and a high-efficiency power amplifying method using the feed-forward amplifier.
Recently multi-carrier radio communication schemes are widespread which permit high-speed transmission through the use of plural narrow-band carriers. As compared with a single-carrier communication, the multi-carrier transmission is less susceptible to fading or some other influences of propagation path variations, and hence it is more robust against delayed waves. Furthermore, the multi-carrier radio communication which use plural narrow-band carriers possesses the advantages of simplifying radio circuitry and relaxing the requirements imposed on the radio circuit used.
The multi-carrier radio communication schemes, which are advantageous for high-speed transmission as mentioned above, have been practiced in microwave communications and a multi-channel access system.
In recent years there has been proposed an OFDM (Orthogonal Frequency Division Multiplexing) radio communication scheme intended for high-speed transmission in the microwave band. In the field of broadcasting the application of the OFDM system to the next-generation digital television is now under study.
These multi-carrier radio communication schemes contain various features, but they have such problems as an increase in out-of-band leakage power due to intermodulation distortion by transmitters and the occurrence of intersymbol interference. The intermodulation distortion by transmitters occurs, for example, in a frequency converter or power amplifier. In particular, the influence of the nonlinearity of the power amplifier is serious. In this reason, the multi-carrier radio communication is indispensable of linear power amplification. As a linear power amplifier, a feed-forward amplifier is well-known as a power amplifier capable of removing the nonlinear distortion. The feed-forward amplifier consists of a distortion detecting loop containing a main amplifier and a distortion canceling loop containing an auxiliary amplifier.
In general, simultaneous amplification of two or more carriers by a power amplifier will generate the intermodulation distortion unless the output backoff of the power amplifier is provided corresponding to PAPR (Peak-to-Average Power Ratio). Accordingly, the power amplifier needs to be sufficiently high in its saturation output power as compared with the average power of the output signal; this degrades the amplification efficiency. The same is true of the main amplifier in the feed-forward amplifier.
The efficiency of the main amplifier can be enhanced by means of a push-pull circuit of Class-B bias condition or the like. The nonlinear distortion by the main amplifier can be compensated for by a conventional feed-forward amplifier. According to, for instance, literature (Toshio NOJIMA and Shoichi NARAHASHI, xe2x80x9cExtremely Low-Distortion Multi-Carrier Amplifier for Mobile Communication Systems,xe2x80x9d Technical Report of IEICEJ Radio Communication System Study Group, RCS90-4, 1990), in the case where the saturation output of the main amplifier is 100 W, the saturation output of the auxiliary amplifier is xe2x85x9 that of the main amplifier. GaAs-MESFETs (Metal Semiconductor Field Effect Transistors) are used as semiconductor amplifying elements of the main and auxiliary amplifiers. If the drain voltage and current of the MESFET of the main amplifier and auxiliary amplifier operated on 1.5-GHz band are 12 V, 20 A, and 12V, 5 A respectively, the drain efficiency of the feed-forward amplifier is about 5% or below under the Class-A bias condition. By using a Class-B push-pull amplifier or similar high-efficiency amplifier circuit and a Class-A amplifier circuit as the main amplifier and the auxiliary amplifier of the feed-forward amplifier, respectively, it is possible to obtain a drain efficiency of approximately 10% or below.
Moreover, enhancement of the power efficiency of the feed-forward amplifier requires further improvement of the drain efficiency of the main amplifier. As a method for performing high-efficiency amplification while achieving linear amplification, there has been proposed a method using a drain voltage control scheme (Koji CHIBA, Toshio NOJIMA and Shigeru TOMISATO, xe2x80x9cBi-Directional Feed-Forward Drain-Voltage-Controlled Amplifier (BDF-DVCA)xe2x80x9d, Technical Report of IEICEJ Radio Communication System Study Group, RCS89-33, 1989). This method increases the drain efficiency by an equivalent reduction of the output backoff of the amplifier using drain voltage control.
The drain voltage control scheme modulates the power to be supplied to such a semiconductor device as a FET. For example, in a base-station power amplifier of a 100-W average output power, if the drain efficiency of a final-stage FET is 50% (which is theoretically maximum under the Class-A bias condition), the power supply to the final-stage FET is 200 W. When the drain voltage of the FET is 10 V, the drain current is 20 A. Control of such a large current is effected by the FET or the like. With higher-output power amplifiers, however, it becomes more difficult to achieve low-loss drain voltage control due to a loss such as an on internal resistance of the FET that controls the large current.
It is therefore an object of the present invention to provide a method that reduces the output backoff of the main amplifier while retaining a distortion compensating ability equal to or higher than that of the conventional feed-forward amplifier, and a high-efficiency feed-forward amplifier using the method.
According to the present invention, there is provided a feed-forward amplifier that has a distortion detecting loop for detecting a distortion component and a distortion canceling loop following to the distortion detecting loop, for canceling the distortion component and amplifies the power of an input transmission signal, wherein:
the distortion detecting loop comprises:
a main amplifier path containing a main amplifier;
a signal power dynamic range compressing circuit inserted in the main amplifier path at the input side of the main amplifier, for compressing the power dynamic range of the input signal;
a first linear signal transfer path;
a first directional coupler for dividing the transmission signal to the main amplifier path and the first linear signal transfer path; and
a second directional coupler for power-combining the output signals from the main amplifier path and the first linear signal transfer path and for dividing the combined output signal to two combined outputs;
the distortion canceling loop comprises:
a second linear signal transfer path supplied with the one combined output from the second directional coupler, for transferring the supplied combined output;
an auxiliary amplifier path containing the auxiliary amplifier and supplied with the other combined output from the second directional coupler, for transferring the supplied combined output; and
a third directional coupler for power-combining the outputs from the second linear signal transfer path and the auxiliary amplifier path and for outputting the power-combined transmission signal; and
the signal power dynamic range compressing circuit comprises:
a fourth directional coupler for dividing a signal on the main amplifier path to two signals and for outputting the two divided signals;
a third linear signal transfer path for linearly transferring the one of the two divided signals from the fourth directional coupler;
a compressing signal generator for generating a compressing signal that makes the envelope of the one divided signal constant based on the other divided signal from the fourth directional coupler; and
a fifth directional coupler for power-combining the output signal from the third linear signal transfer path and the compressing signal from the compressing signal generator and for providing the power-combined output to the main amplifier.